When hot meets cold: weather-induced tides may impact the Louisiana coast

BATON ROUGE - Astronomical tide, the steady in – and – out of water caused by the moon and sun’s gravitational forces, has a relatively small impact on the Louisiana coast. Tide in this area of the northern Gulf is classified as “micro-tidal”, with a maximum range of only about 60 centimeters of difference between high and low water levels.

However, an entirely different type of tidal action may prove to be a significant factor. Meteorological tide, the rise and fall of water levels based on the repeated passing of synoptic weather systems, may have a greater impact on processes like coastal erosion and sediment transport, says Chunyan Li, a professor in the Department of Oceanography & Coastal Sciences.  

Li said meteorological tides differ quite a bit from their astronomical counterparts.  “Meteorological tide is relatively stronger in this region with irregular occurrence and strength,” and can be difficult to predict, as it is caused by large scale atmospheric conditions.  He noted weather-caused tides can also be powerful. “This forcing is generally larger than tidal fluctuation in this region during cold seasons and can be hazardous anywhere with extreme events.”

Li and fellow DOCS Professor Robert Rohli recently published a paper illustrating the impact meteorological tide can have. The paper details the conditions that occurred as a result of Tropical Storm Lee passing over the Louisiana coast in 2011. While Lee was not known for its strong wind conditions on its own, when it made landfall over Vermilion Bay, it collided with a cold front as the leading edge of a cold air mass dipping down from Montana.

Li had acoustic monitoring equipment in the bay and offshore platforms. His equipment was able to fully capture wind conditions, wave action and atmospheric pressure, providing him with a detailed picture of the impact weather-created tides can have.

When they met, the two storms fed off one another, reenforcing the cyclonic wind patterns and water movement and forcing large amounts of water from the bay.   At the height of the storm, tidal currents in the narrow Southwest Pass of Vermilion Bay increased from one meter per second to 2.4 meters per second.    

In the paper, Li notes the conditions captured “highlight the significant implications of such weather interactions for coastal erosion, sediment transportation, and sediment deposition during coastal weather events.”

The Rise of Cold Fronts

It is well documented that when storm systems interact, they can complicate each others’ impacts. Superstorm Sandy, in 2012, is one such case. Subtropical and extra-tropical systems met, causing ocean waves exceeding 7 meters and eroding 3 cm of continental shelf.

However, the collision of the two weather systems that Li’s sensors picked up is a little different. They involve a tropical cyclone meeting a cold front, something that has not been as well-studied.

Li, who has studied meteorological and hydrodynamic conditions in the northern Gulf for twenty years, says that an average of more than forty cold fronts pass over the coast every year, making these sorts of collisions, and the meteorological tides they create, more likely. He also noted that the number has increased slightly in the last half century, even as the average global temperatures rise.

“Recurring cold fronts act like a piston, generating significant meteorological tides and driving the movement of water, sediments, and nutrients. Climatic fluctuations can alter this forcing, continuously shaping both the coastline and its ecosystems,” said Li.